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Axial lung window
The image above is a good example of paraseptal emphysema. There is also a background of centrilobular emphysema.
Paraseptal emphysema involves the alveolar ducts and sacs at the lung periphery. The emphysematous areas are often surrounded by interlobular septa (hence the name). It may be an incidental finding in young adults, and may be associated with spontaneous pneumothorax. It may also be seen in older patients with centrilobular emphysema (as in this case). Both centrilobular and paraseptal emphysema may progress to bulla formation.
A bulla is defined as being at least 1 cm in diameter, and with a wall less than 1 mm thick. Bullae are thought to arise by air trapping in emphysematous spaces, causing local expansion.